The present invention relates to a method for determining a characteristic variable which relates to the state of charge of a storage battery.
It may be desirable to determine or to predict at any given time the state of an electrochemical storage battery (e.g., the state of charge or the heavy-current load capacity).
By way of example, the capability of a starter battery to start a motor vehicle with an internal combustion engine is governed by the state of charge and the state of aging, or by the capacity drop which has occurred, of the battery, since the current level which can be drawn from the starter battery and the power which can be emitted are limited. It may be desirable to determine the state of charge or the starting capability of a starter battery in situations in which, for example, the engine is operated intermittently, since, in this case, the vehicle electrical power supply system, together with its loads, is still operated in periods in which the engine is switched off, although the generator is not producing any electrical power. In cases such as this, the monitoring of the state of charge and the starting capability of the storage battery must ensure the energy content of the storage battery always remains sufficient to still start the engine.
Widely differing methods are known for measurement of the state of charge of storage batteries. By way of example, integrating test equipment (amp-hour (Ah) meters) is used for this purpose, with the charging current being taken into account, and possibly being weighted with a fixed charging factor. Since the usable capacity of a storage battery is highly dependent on the magnitude of the discharge current and on the temperature, even methods such as these may not allow a satisfactory statement to be made about the usable capacity which can still be drawn from the battery.
By way of example, in the case of a method for measurement of the state of charge, it is known from DE 22 42 510 C1 for the charging current to be weighted with a factor which is itself dependent on the temperature and on the state of charge of the battery.
DE 40 07 883 A1 describes a method in which the starting capability of a storage battery is determined by measurement of the battery terminal voltage and the battery temperature, and by comparison with a state of charge group of characteristics which is applicable to the battery type to be tested.
DE 195 43 874 A1 discloses a calculation method for the discharge characteristic and remaining capacity measurement of a storage battery, in which the current, voltage and temperature are likewise measured, with the discharge characteristic being approximated by means of a mathematical function with a curved surface.
DE 39 01 680 C1 describes a method for monitoring the cold starting capability of a starter battery, in which the starter battery is loaded with a resistance at times. The voltage which is dropped across the resistance is measured, and, in comparison with empirical values, it is used to determine whether the cold starting capability of the starter battery is still adequate. The starting process is in this case used to load the starter battery.
Furthermore, DE 43 39 568 A1 discloses a method for determination of the state of charge of a motor vehicle starter battery, in which the battery current and a rest voltage are measured, and from which the state of charge is deduced. In this case, the battery temperature is also taken into account. The charging currents which are measured during different time periods are compared with one another, and a remaining capacity is determined from them.
DE 198 47 648 A1 describes a method for learning a relationship between the rest voltage and the state of charge of a storage battery for the purpose of estimation of the storage capability. A measure for the acid capacity of the electrolyte of the storage battery is determined from the relationship between the rest voltage difference and the amount of current transferred during the load phase. In this case, use is made of the fact that the rest voltage rises approximately linearly with the state of charge in the higher state of charge ranges which are relevant in practice.
One problem of determining the state of an electrochemical storage battery with known methods is that wear factors which are not all relevant are taken into account, particularly when acid stratification is present.
In the case of a lead-acid rechargeable battery, the electrolyte is composed of dilute sulfuric acid, that is to say, a solution of H2SO4 in water. Typically, this is an approximately 4 to 5 molar solution when in the completely charged state. During the discharge reaction, H2SO4 in the electrolyte is consumed at both electrodes in accordance with the reaction equations:Positive electrode: PbO2+H2SO4+2H++2e−→PbSO4+2H2ONegative electrode: Pb+H2SO4→Pb+2H++2e−and, furthermore H2O is formed at a positive electrode. The concentration and the specific gravity of the electrolyte thus fall during discharging, while they rise again during the charging reaction, which takes place in the opposite manner.
If the sulfuric acid which is formed during the charging reaction has the capability to be convected in the field of gravity of the earth, then it has the tendency to fall in layers to the bottom of the cell vessel of the lead-acid rechargeable battery cells. An electrolyte in a higher concentration is then located in the lower area of the respective cell vessel than in the upper area of the cell vessel. In the case of a lead-acid rechargeable battery, this state is referred to as acid stratification.
Since both the charging reaction and the discharge reaction as well as the parasitic reactions, such as gas development, corrosion etc., are in general influenced by the electrolyte concentration, acid stratification leads to nonuniformity in the state of the cell.
It would thus be advantageous to provide an improved method for determination of a characteristic variable which relates to the state of charge of a storage battery.